Electrochemical conversion of phenol to hydroquinone

ABSTRACT

The electrochemical conversion of phenol to hydroquinone has been improved by a combination of steps directed to effect removal of by-products and conversion of p-benzoquinone to hydroquinone as part of the process for the recovery of the hydroquinone product. These steps include first reducing residual p-benzoquinone to hydroquinone, followed by removal of tars and color bodies, vacuum distillation to condense the volume of the electrolyzed reaction mixture, and finally crystallizing the hydroquinone product out of the condensed distillation residue.

United States Patent Covitz et a1.

1 5] 3,663,381 [451 May 16,1972

[54] ELECTROCHEMICAL CONVERSION OF PHENOL TO HYDROQUINONE [72] Inventors: Frank H. Covitz, Lebanon; Robert V. Carrubba, Cranford, both of NJ.

[73] Assignee: Union Carbide Corporation, New York,

[22] Filed: Apr. 9, 1970 [21] Appl. N0.: 26,924

[52] US. Cl ..204/78 [5 1 Int. Cl. .C07b 29/06, C07c 37/00 [581 Field of Search ..204/78 [56] References Cited UNITED STATES PATENTS 2,130,151 9/1938 Palfreeman ..204/78 2,135,368 11/1938 Vagenius et al. ....204/78 3,509,031 4/1970 Covitz ..204/78 so; FLOW TANK METER OVERFLOW FROM OTHER PUBLICATIONS Laboratory Practice of Organic Chem. by Robertson pp 77, 78 pub by Macmillan Co., New York 1939 Primary ExaminerF. C. Edmundson AttorneyPau1 A. Rose, Aldo J. Cozzi and Bernard F. Crowe [5 7] ABSTRACT The electrochemical conversion of phenol to hydroquinone has been improved by a combination of steps directed to effect removal of by-products and conversion of p-benzoquinone to hydroquinone as part of the process for the recovery of the hydroquinone product. These steps include first reducing residual p-benzoquinone to hydroquinone, followed by removal of tars and color bodies, vacuum distillation to condense the volume of the electrolyzed reaction mixture, and finally crystallizing the hydroquinone product out of the condensed distillation residue.

Claims, 1 Drawing Figure 28 CONDE NSER OVERFLOW ELECTROLYSIS LEVEL CELL CONTROLLER 32 T0 TRAPS 1 v -AND VACUUM 1 SOURCE I6 OVERHEAD 502 REACTOR HOLDING TANK "4 CHARCOAL COLUMN STEAM DRAIN TEMPERATURE CONTROLLER 1 1 g OVERFLOW TO 46' I 36' DRAIN O 44 P l 48 I k BOTTOMS HOLDING TANK ELECTROCHEMICAL CONVERSION OF PHENOL TO HYDROQUINONE BACKGROUND OF THE INVENTION This invention relates to the electrochemical conversion of phenol to hydroquinone and particularly to an improved method of recovery of the hydroquinone product.

The electrochemical oxidation of phenol to produce hydroquinone has been shown to be feasible by controlling such variables as the weight per cent of phenol, the weight percent of electrolyte used, the temperature of the electrolysis, the pH of the aqueous solution, the voltage used, the current density used, and the control of the per cent conversion of phenol to hydroquinone. As is the case with many electrolysis reactions, the electrochemical oxidation of phenol to hydroquinone is characterized by the formation of a plurality of products some desirable, some undesirable. If this reaction were ideal only quinone would be produced at the anode and all of this would then be reduced to hydroquinone at the cathode. However, the reaction is far from ideal and so there is obtained in addition to the desired products described above such undesired by-products as carbon monoxide, carbon dioxide, oxalic acid, maleic acid, tars, and other unidentified material. The tars are believed to arise through secondary reactions of the above described products. A possible sequence of reactions leading to tars consists in the reaction of p-benzoquinone with phenol in the presence of hydrogen ion to product phenoxy hydroquinone, the reaction of p-benzoquinone with water to produce hydroxy hydroquinone and the reaction H 1 l l henol OH i II lit) a i II on of p-benzoquinone with hydroquinone to producehydroxy phenoxy hydroquinone. These reactions are delineated above. These intermediates are sub'ect to further side reactions until highly complex and highly colored compounds or tars are formed. These undesired reactions not only reduce the chemical and electrical efficiencies of the basic electrochemical oxidation of phenol but also exert further deleterious effects in the recovery of the hydroquinone product from the reaction mixture efiluent. It was unexpectedly found that the undesired side-reaction products in this electrolysis operation exert a definite and measurable effect on the stability of the hydroquinone product during its isolation. The mechanism for this phenomenon is unknown but the results are very real. It is therefore desired that by-products be eliminated as soon as possible in the recovery of the hydroquinone products.

SUMMARY OF THE INVENTION It has now been discovered that in the method of preparing hydroquinone from phenol which comprises the steps of:

llydroquinonc IIO-Q-O- a. electrolyzing an aqueous solution containing from about 0.5 to 4 percent by weight of phenol and about 1 to 5 percent by weight of sulfuric acid at a temperature of about 25 to 100 C., a pH of less than about 4, an anode d.c. potential of at least about +0.9 volts in reference to a saturated calomel electrode, a cathode d.c. potential more negative than +0.4 volts in reference to a saturated calomel electrode, and a current density of at least 4 amperes per square decimeter until up to about percent by weight of the phenol has been electrolyzed to hydroquinone; and,

b. recovering the hydroquinone from the aqueous solution, the isolation of hydroquinone product can be improved by the steps of:

c. treating the electrolyzed aqueous solution from step (a) with sufficient reducing agent to convert p-benzoquinone to hydroquinone;

d. removing tars and color bodies;

e. vacuum distilling at a pot temperature of about 45-65 C. and a pressure of about 70 to about 200 millimeters of mercury until the volume of the electrolyzed aqueous solution is about one-fourth to one-tenth that of the original volume;

f. allowing the distilled pot residue to cool one crystallizes out of solution; and,

g. recovering the crystallized hydroquinone.

DESCRIPTION OF THE PREFERRED EMBODIMENT It is preferred that the steps delineated supra be carried out in the order specified therein.

Conventional vacuum distillation equipment can be used for the stripping operation used for concentrating the electrolyzed aqueous solution after the reduction and tar removal and color body removal steps have been carried out.

The temperature range used for the stripping operation has a critical upper limit in that temperatures above 65 result in decomposition of the hydroquinone product as evinced by the formation of additional color bodies. While the lower temperature limit of 45 is not narrowly critical, temperatures below this point brings the stripping operation to a point where economic factors become a consideration due to the higher vacuum needed.

The pressure range is critical in that it corresponds to pressures at which the temperature range is feasible.

It was completely unexpected that the electrolyzed aqueous solution could be concentrated to as 'low a volume as onetenth that of the original volume because the distillate consists mainly of water and phenol and results in an increase in the acid concentration of the stripped residue containing the hydroquinone product to about 10 times that of the feed stream. Since sulfuric acid is commonly used as the electrolyte both because of its effectiveness and low cost, this would mean the effective sulfuric acid concentration increases to about 30 percent. Since concentrated sulfuric acid is notoriously reactive towards organic compounds, it was completely unexpected that no decomposition of the hydroquinone resulted in the recovery process. This fortuitous discovery not only permits the recycle of sulfuric acid but the presence of the sulfuric acid after this concentration step aids in the crystallization of the hydroquinone product from the distillation residue by a salting out effect.

It was also inadvertently discovered that certain unknown agents produced in the electrolysis of phenol which enhance or catalyze the decomposition of hydroquinone are removed by the reducing and adsorption treatment of the effluent. This was demonstrated by first preparing a synthetic aqueous s0lution Control consisting of 1.5 percent phenol, 0.9 to 1.3 percent hydroquinone, 0.2 to 0.5 percent p-benzoquinone and 3 percent sulfuric and subjecting this solution to a distillation at atmospheric pressure until hydroquinone could be crystallized out of the pot residue. A quantitative recovery of hydroquinone light tan in color was thus effected. In contrast when the until hydroquinelectrolysis cell effluents containing 1.5 percent phenol, 0.9 to 1.4 percent hydroquinone and 0.5 p-benzoquinone and about 3 percent sulfuric acid were subjected to distillation at atmospheric pressure until hydroquinone crystallized out, the

product was black and only a 50 percent recovery of 5 hydroquinone could be achieved. Only when the electrolysis cell effluent was subjected to the combined steps of I) treatment with a reducing agent (preferably 80,) (2) removing tar and color bodies by contacting with charcoal (or equivalent adsorbent) and distillation under reduced pressure were results similar to the Control achieved i.e., quantitative recovery of lightly colored hydroquinone. The omission of any one of these steps produced deleterious results.

The hydroquinone which crystallizes out can then be recovered from the aqueous acid supernatant solution by any technique well known in the chemical art such as filtration centrifugation and the like. The temperature of crystallization is not narrowly critical. For example, ambient room temperature is most convenient although temperatures from about -20 C. shorten the time required for crystallization.

It is preferred to use sulfur dioxide as the reducing agent in step (c) of the improved process delineated above, although other reducing agents can be used if desired, e.g., nascent hydrogen which can be generated in situ by adding a metal, higher in the electrochemical series than hydrogen, to the acidic effluent from the electrolysis cell. Examples of suitable metals include zinc, tin, iron and the like. A further modification consists in effecting reduction electrolytically at the cathode of a divided cell instead of by the use of a reducing agent. A unique advantage in the use of sulfur dioxide lies in the fact that it is converted to sulfuric acid in the reduction process and thus forms more of this useful electrolyte instead of a foreign substance or contaminant.

The preferred method for removal of residual tars and color bodies entails the use of an adsorbent grade of activated charcoal. It has been found convenient to utilize this adsorbent in the form of a column allowing the clectrolyzed aqueous solution to either drip through by gravity or perculate through. Other adsorbents which can be used for this step include: activated alumina, molecular seives and the like. A particularly preferred decolorizing agent is activated cocoanut charcoal having a high surface area, used in the form of a powder or granules in the range of about 40 to 200 mesh. I

It is preferred to use a continuous operation in which a feed of 3 percent phenol and 3 percent sulfuric acid is fed as an aqueous solution to the electrolysis cell at a rate sufficient to afford a phenol conversion of about 50 percent by weight. Other preferred conditions include a current density of about 20-40 amperes per square decimeter, an electrolysis temperature of about 50-60 C. and the use of an expanded lead anode preanodized in 30 percent aqueous sulfuric acid and a cathode of aluminum, amalgamated lead or Monel metal (Trademark of the International Nickel Co., Inc. for a wrought nickel-copper alloy containing approximately twothirds nickel and one-third copper). In this preferred embodiment the stripping operation affords an overhead distillate consisting of phenol and water which is available for recycle to the electrolysis cells. It is particularly preferred to employ a series of electrolysis cells connected in series, in place of a single electrolysis cell, starting with undivided cells and finishing with divided cells.

Hydroquinone and p-benzoquinone were analyzed for in aqueous solutions by known polarographic techniques recognized in the art. One ml. samples were withdrawn from the electrolysis cell and transferred to a 25 ml. volumetric flask and the liquid meniscus brought to the fiducial mark with a 0.2 molar pH 7 aqueous phosphate buffer. A polarogram of each solution was obtained and the diffusion limiting currents for hydroquinone and p-benzoquinone were determined. These data were compared with a calibration curve prepared from standard hydroquinone and p-benzoquinone solutions. The

calibration curves consisted of a plot of diffusion limiting current in microamperes versus concentration in moles or grams.

The phenol content of aqueous solutions was determined by vapor phase chromoiography. Test samples were treated with excess sulfur dioxide and extracted with an equal volume of amixture of 98 percent toluene and 2 percent dichlorobenzene. (as an internal standard) in a thermostat at 530 C. A small sample of the toluene phase was injected into the vapor phase chromotography apparatus operating with a 2 meter column containing 10 percent solid polyethylene oxide deposited on a Teflon (Trademark for polytetrafluoroethylene) support at 1 C.

Calibration was effected against a standard phenol solution by the same technique. Duplicate analyses were performed in each case. Measurement consisted of calculation of the integrated area of the phenol peak and the integrated area of the internal standard and comparison with a calibration chart prepared from a standard phenol solution by the same technique.

Acids were determined by non-aqueous titration of 1 ml. samples from the electrolysis cell (diluted with 25 ml. of isopropyl alcohol) with about 0.1N standardized tetramethylammonium hydroxide in methanol. An automatic titrator with an external derivative and logarithmic response transducer was used to generate end-point peaks.

Orsat analysis was used to determine C0 ,CO, and 0 with H, determinations by difference.

The invention is further described in the example which follows. All parts and percentages are by weight unless otherwise specified.

EXAMPLE A continuous electrolysis reactor cell was constructed consisting of a circular lead anode and circular lead cathode separated by an insulating collar or cell spacer 1.6 centimeters thick of polypropylene having an inlet and outlet means for delivery and removal of the reactants to and from the electrolysis cell. The cell also is fitted with a thermocouple well and a connection to the reference electrode of a polarographic apparatus. The effective electrode area of both the anode and the cathode was 25 centimeters sq. The lead electrode surfaces prior to the actual electrolysis experiments were first scoured with 400 mesh silicon carbide sandpaper, polished with crocus cloth and then washed with water. These lead electrodes were then pressed to a fiat polished surface between chrome plated steel plates. The electrodes were weighed prior to cell assembly. For better reproduceability from run to run a standard preconditioning of the electrodes was carried out consisting of pre-electrolyzing them by electrolyzing a 3 percent sulfuric acid solution for 30 minutes at a current density of 40 amperes per sq. dec. with 10 amperes passing through the cell. After this preconditioning a standard aqueous feed composition consisting of 3 percent phenol, 3 percent sulfuric acid (wt./vol. per cent) was fed into the electrolysis cell maintained at a temperature of 50 C. at a feed rate of 4.95 ml. per minute. The effluent from the cell was obtained by overflow and thus the withdrawal rate was the same as the feed rate of 4.95 ml. per minute. The recovery system is best described by referring to the FIGURE where the overflow from the cell is shown in a flow diagram as passing into the SO, reactor 1 which is simultaneously saturated with S0 from tank 2 through a flow meter 3. The S0; reactor 1 also has an overflow takeoff level which maintains a constant level in the S0 reactor 1 and leads the overflow into a charcoal column 4 which is standard chromatagraphic glass column 0.5 inches in diameter and 18 inches high filled to a level of 16 inches with 40 mesh adsorbent grade charcoal. The charcoal column 4 is equipped with a level overflow drain 6 and is connected to a level controller 8 through a sensor 10. The purified and reduced effluent is removed through the bottom of charcoal column 4 by means of line 12 which leads to flow rate valve 14 and thence to solenoid valve 16. The solenoid valve 16 is activated or deactivated by level controller 8. When the solenoid valve 16 is opened the treated effluent passes into a stripping column 18 which is heated through steam passing through valve 20 into jacket 22. Steam is drained from jacket 22 through drain 24. The distillate taken off at the top of stripping column 18 through line 26 passes to a condenser 28. The condensate consisting mainly of water and unreacted phenol is stored in overhead holding tank 30 which in turn is connected to traps and a vacuum source through line 32. The bottoms emerging from the stripping column 18 through line 34 into tank 36 contained mainly the product hydroquinone and concentrated aqueous sulfuric acid. The level in tank 36 is controlled through sensor 38 connected to level controller 40 which in turn activates solenoid valve 42. The temperature of the bottoms in tank 36 is measured by thermocouple 44 which is connected to temperature controller 46. Temperature controller 46 activates the steam control valve 20. The product removed from tank 36 passes through recycle pump 48 and then either to bottoms holding tank 50 or back to the stripping column 18 through line 52. The bottoms holding tank 50 is connected to the vacuum source and traps through line 54.

In a representative experiment using the apparatus described above, hydroquinone was produced at a rate of 0.0301 moles per hour (3.25 grams per hour) and pbenzoquinone at a rate of 0.0101 moles per hour (1.07 grams per hour). This represents a ratio of hydroquinone to pbenzoquinone of 3.0. The electrical efficiency for this experiment was 43 percent and the chemical efficiency 85 percent.

The recovery operating conditions used included an input rate into the S0 reactor 1 of 4.95 ml. of electrolysis cell overflow per minute with an $0 flow rate into SO reactor 1 of 5.0 ml. per minute at standard temperature and pressure. The pot temperature in stripping column 18 was maintained at 50 C. while the pot pressure in stripping column 18 was maintained at about 90 mm of mercury Hg. The overhead to bottoms ratio in stripping column 18 was about 8.0. The bottoms takeoff rate was about 0.62 ml. per minute. The product in the bottoms holding tank 15 was placed in a crystallization container at about 0 C. in which an 80 percent recovery of hydroquinone was effected at a crystallization rate of about 3.48 grams per hour.

The distillate obtained as the overhead fraction from stripping column 18 which collected in overhead holding tank 30 and which consists mainly of water and phenol can be recycled to the electrolysis cell if desired with sufficient phenol and sulfuric acid to make up the original charge.

Although the invention has been described in its preferred forms with a certain degree of particularity, it is understood that the present disclosure has been made only by way of example, and that numerous changes can be made without departing from the spirit and scope of the invention.

What is claimed is:

1. In the method of preparing hydroquinone from phenol which comprises the steps of:

a. electrolyzing an aqueous solution containing from about 0.5 to 4 percent by weight of phenol and about I to 5 percent of sulfuric acid at a temperature of about 25 to C., a pH of less than about 4, an anode d.c. potential of at least about 0.9 volts in reference to a saturated calomel electrode, a cathode d.c. potential more negative than about 0.4 volts in references to a saturated calomel electrode, and a current density of at least 4 amperes per sq. dec. until up to about 50 percent by weight of the phenol has been electrolyzed to hydroquinone and b. recovering the hydroquinone from the aqueous solution, the improvement which consists essentially of carrying out in order the steps of:

c. treating the electrolyzed aqueous solution from step (a) with a sufficient reducing agent to convert p-benzoquinone to hydroquinone;

d. removing tars and color bodies with an absorbent selected from the group consisting of activated alumina, activated charcoal and molecular sieves;

e. vacuum distilling at a pot temperature of about 45 to 65 C. and a pressure of about 70 to about 200 mm. of mercury until a volume of the electrolyzed aqueous solution is about one-fourth to one-tenth that of the original volume;

f. allowing the distilled pot residue to cool until hydroquinone crystallizes out of solution; and

g. recovering the crystallized hydroquinone.

2. The method claimed in claim I wherein the phenol concentration is about 3 percent and the sulfuric acid concentration is about 3 percent.

3. Method claimed in claim 2 wherein the electrolysis temperature is about 50 to 60 C. and the current density is about 20 to 40 amperes per square decimeter.

4. Method claimed in claim 1 wherein the reducing agent in step (c) is sulfur dioxide.

5. Method claimed in claim 1 wherein the tars and color bodies are removed in step (d) by activated charcoal.

6. Method claimed in claim 1 wherein the electrodes are preconditioned by pre-electrolyzing at 10 amperes, a current density of 40 amperes per square decimeter, and room temperature in a 3 percent aqueous sulfuric acid solution.

7. Method claimed in claim 1 wherein the anode and cathodes are both fabricated from lead.

8. Method claimed in claim 1 wherein the anode is fabricated from lead and the cathode from amalgamated lead.

9. Method claimed in claim 1 wherein the anode is fabricated from lead and the cathode from aluminum.

10. Method claimed in claim 1 wherein the anode is fabricated from lead and the cathode from Monel metal. 

2. The method claimed in claim 1 wherein the phenol concentration is about 3 percent and the sulfuric acid concentration is about 3 percent.
 3. Method claimed in claim 2 wherein the electrolysis temperature is about 50* to 60* C. and the current density is about 20 to 40 amperes per square decimeter.
 4. Method claimed in claim 1 wherein the reducing agent in step (c) is sulfur dioxide.
 5. Method claimed in claim 1 wherein the tars and color bodies are removed in step (d) by activated charcoal.
 6. Method claimed in claim 1 wherein the electrodes are preconditioned by pre-electrolyzing at 10 amperes, a current density of 40 amperes per square decimeter, and room temperature in a 3 percent aqueous sulfuric acid solution.
 7. Method claimed in claim 1 wherein the anode and cathodes are both fabricated from lead.
 8. Method claimed in claim 1 wherein the anode is fabricated from lead and the cathode from amalgamated lead.
 9. Method claimed in claim 1 wherein the anode is fabricated from lead and the cathode from aluminum.
 10. Method claimed in claim 1 wherein the anode is fabricated from lead and the cathode from Monel metal. 